The transverse computer assisted tomography (CT) method of calculating ventricular volumes of the human brain in vivo, using TRACE image analysis, procedure was found to be highly reliable, and suitable for longitudinal studies of aging and dementia. Quantitative CT analyses in healthy aging men and women demonstrated significant sex differences in ventricular volume and in age of onset of ventricular enlargement. Structural brain changes, as measured by ventricular enlargement, and decline in cognitive performance on the WAIS appear to be relatively independent processes correlated more to the age of the subject. Future studies using magnetic resonance imaging (MRI) will be devoted to analyzing other structures which may change with age and explain the mild cognitive decline seen in healthy aging, and will form the basis for comparison to patients with dementia of the Alzheimer's type (DAT). Three sets of monozygotic twins discordant for DAT were studied and found to have significant differences in cross sectional ventricular volumes. Moreover, the DAT patients were found to have greater than two standard deviation increased rates of ventricular enlargement whereas their unaffected twins were within the normal range. Down syndrome (DS) patients with dementia were also shown to have a significantly increased rate of ventricular enlargement when compared to young and nondemented older DS patients.